Updated binder link and logo

Author: nthiery

README.rst

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 A SageMath-based computing environment for binder
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-.. image:: https://beta.mybinder.org/badge.svg
-   :target: https://beta.mybinder.org/v2/gh/sagemath/sage-binder-env/master?filepath=index.ipynb
+.. image:: https://mybinder.org/badge_logo.svg
+ :target: https://mybinder.org/v2/gh/sagemath/sage-binder-env/master?filepath=index.ipynb
 
 This repository illustrates Binder use cases for
 `SageMath <http://sagemath.org>`_.